This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. P2001-81595, filed on Mar. 21, 2001; the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an electrophoresis display.
The invention is based on the guest-host effect which is realized by the switching of a dichroic coloring matter dissolved in a liquid crystal on applying voltage, and on the electrophoresis phenomenon of electrophoretic fine particles dispersed in a dispersion medium.
2. Discussion of the Background
The electric field inductive pigments of electrophorectic type used in the electrophoresis display at present are roughly classified in two types.
One of them is the type 1 shown in FIG. 7A. This type of pigment comprises microcapsules which enclose a coloring solvent 12. The coloring solvent 12 contains charged fine titania particles 11 dispersed therein. By using the electric field inductive pigment of this type, two colors, i.e., the white color of the fine titania particles 11 and the color of the solvent, can be displayed.
The other type of the electric field inductive pigment is the type 2 shown in FIG. 7B. This type contains a transparent solvent 22 enclosed in the microcapsules. Two types of charged fine particles 21a and 21b differing in charge sign and in color are dispersed in the transparent solvent 22. For instance, white colored positive charged fine particles 21a and black colored negative charged fine particles 21b are used. When negative voltage is applied to the upper side of the drawing, the white colored charged fine particles 21a concentrate to the upper side as shown in FIG. 7B. On the other hand, when a positive voltage is applied to the upper side, the black colored charged fine particles 21b concentrate to the upper side. That is, the type of fine particles appearing on the surface can be selected by controlling the direction of the electric field applied to the microcapsule, and the dichroic display is thereby realized.
The following literatures describe the above.
[1] B. Comiskey, J. D. Albert and J. Jacobson, Digest of SID97, p75.
[2] P. Drzaic, B. Comiskey, J. D. Albert, L. Zhang, A. Loxley and R. Feeney, Digest of SID99, p1131.
[3] Barrett Comiskey, J. D. Albert, Hidekazu Yoshizawa and Joseph Jacobson, Nature, 394, p253 (1998).
Although a display using the electric field inductive pigment above is advantageous in that they yield high contrast, it suffers a problem that there is difficulty in providing color display.
More specifically, in case of an electric field inductive pigment of type 1, the display is provided in two types, i.e., a white display using the scattering of fine titania particles 11 and a color display using the color of the dispersion medium 12. Hence, a color filter is necessary to realize a color display. In case of an electric field inductive pigment of type 2, the display is limited to two colors depending on the fine electrophoretic particles 21a and 21b. Hence, a color filter is indispensable in this case again.
FIG. 8 shows the method for realizing a color display by applying a color filter together with the electric field inductive pigment. In this case, the microcapsules, which contain a black colored solvent 12 having the white colored charged fine particles 11 dispersed therein, are densely arranged on the substrate. Red, green, and blue color filters are provided aligned to the position of each of the microcapsules. In case of realizing a red display, for instance, the green and the blue pixels are set in the light absorbing state (black display), while the red pixels alone are set in the light reflecting state. The intensity of the reflection light is reduced to about one third due to the absorption of the color filter 13, and, the resulting intensity is further reduced to about one third because the green and blue pixels turn to the light absorption state. Hence, the resulting optical efficiency becomes as low as about {fraction (1/9)} (⅓xc3x97⅓={fraction (1/9)}).
Further, in case of realizing a white color display, the red, green, and blue pixels all turn to a light reflecting state, but due to the optical absorption of the color filter, the resulting display is reduced to about ⅓ of the incident light intensity despite the reflectance is at the maximum.
Accordingly, it is presumed that only a dark display is realized by the color filter type. Since an increase in optical transmittance of the color filter is in trade off with the improvement in color purity of the display color, the range of color reproduction decreases in an attempt to improve the transmittance of the color display by increasing the optical efficiency.
On the other hand, there is proposed a method of coloring the fine particles themselves in the three color primaries. More specifically, as shown in FIG. 9, this method comprises coloring the electrophoretic fine particles 31, and pattern printing the color region.
In accordance with the method, a white display is realized by additive color mixture of red, green, blue colors, but it is a dark display with an optical efficiency of about ⅓.
Furthermore, as shown in FIG. 10, there is a method of realizing color display by using a colored solvent. This method comprises coloring the dispersion medium of the white colored electrophoretic fine particles 11 into red 41, green 42, and blue 43 colors. For instance, in case of red display, the red microcapsules are set to the light absorbing state, while the green and blue microcapsules are set to the light reflecting state. Although a bright color display can be obtained in this case, the display results in an unclear pale color display. A high reflectivity is obtained in white display, and, in the black display, an additive color mixture state of red, green, and blue is realized with high reflectivity and low contrast.
As shown in FIG. 11, there is a method of realizing a color display by using the fine particles 31 colored in three primaries and the color solvents 41, 42, and 43. That is, this method comprises coloring both of the electrophoretic fine particles and the solvents. In this case, the colors of the solvents and the electrophoretic fine particles are set in the complementary relation with each other to obtain the black display and the colored display with a single capsule. Referring to FIG. 11, three types of microcapsules, i.e., the microcapsules containing red color fine particles with a cyan colored solvent 41, the microcapsules containing green color fine particles with a magenta colored solvent 42, and the microcapsules containing blue fine particles with a yellow colored solvent 43, are densely arranged on the substrate. In a microcapsule containing red color fine particles with a cyan colored solvent 41, as shown in FIG. 11, red color is displayed in case the red fine particles are present in the upper side. On the contrary, when the red fine particles are disposed on the lower side, the color of the fine particles is mixed with the color of the solvent to display a black color. In a microcapsule containing green color fine particles with a magenta colored solvent 42, green color and black color are displayed in case the particles are disposed on the upper side and the lower side, respectively. Further, in a microcapsule containing blue color fine particles with a yellow colored solvent 43, blue color and black color are displayed in case the particles are disposed on the upper side and the lower side, respectively.
For instance, in case of red color display, as shown in FIG. 11, the pixel on the left edge yields a red display, and the other two types of pixels yield a black display. The optical efficiency in this method is about {fraction (1/9)} to result in a dark color display.
In case of white display, red, green, and blue displays are realized with three types of capsules to implement white color with the additive color mixture thereof. However, the optical efficiency is about ⅓ to result in a dark display.
In this method again, there still remains a problem that the reflectance of the white display is decreased and that the reflectance of black display is increased, as to result in a low contrast.
On the other hand, concerning the relation of complementary colors, in case the colors of the colored fine particles and the solvent are exchanged with each other, the optical efficiency in white display increases to about ⅔ to result in a bright display. However, there occurs a problem that the range of color reproduction is greatly limited in the color display due to the planar arrangement of YMC.
In addition to above, there is a method of realizing color display by dispersing two types of electrophoretic fine particles differing in the sign of charges in a transparent solvent. This method can be furthermore classified into a case using a color filter and a case using colored particles.
Firstly, the method using a color filter is explained below by making reference to FIG. 12.
In case of red display, the microcapsules reflect only ⅓ of the incident light, and the color filter 13 absorbs about ⅓ thereof; hence, in total, the optical efficiency in displaying red color results as low as about {fraction (1/9)}, i.e., a dark display is realized. In the case of white display, although all the pixels are in the light reflecting state, the optical efficiency remains low at about ⅓.
Then, the method comprising dispersing two types of electrophoretic fine particles with different charge signs in a transparent solvent 22 is described below with reference to FIG. 13. For instance, a pattern is formed by using three types of capsules; i.e., capsules containing black and red fine particles 51, capsules containing black and green fine particles 52, and capsules containing black and blue fine particles 53.
Referring to FIG. 13, the capsule in the left end contains red fine particles and the black fine particles on the upper side and the lower side, respectively. In the other capsules, the black fine particles are disposed on the upper side, while the green fine particles and the blue fine particles are present on the lower side. In this case, red color is displayed.
In the case of red display, the other two types of capsules yield black displays, and the optical efficiency in displaying color is about {fraction (1/9)}. White display is realized by the additive color mixture of the reflection lights of red, green, and blue, thereby resulting in a dark display with an optical efficiency of about ⅓.
Furthermore, there is another method comprising densely arranging three types of capsules on the substrate while changing the color of the fine particles, i.e., by using capsules containing white and red fine particles 61, capsules containing white and green fine particles 62, and capsules containing white and blue fine particles 63. This case is shown in FIG. 14.
In case of displaying red color with the constitution above, the red fine particles 61 in the left end capsule are concentrated to the upper side, while the white fine particles are disposed to the lower side. The other two capsules are set to display white. Thus, this case results in a pale display with low color purity. A bright display can be realized in the white display because all of the pixels are in the light reflecting state; however, in the black display, the contrast becomes low because it is realized by the subtractive color mixture of the reflecting light of red, green, and blue.
As described above, there are two types of microcapsule type electrophoretic E-ink; a display type comprising switching the color of the electrophoretic fine particles and the solvent, and a display type comprising color switching of the two types of electrophoretic fine particles. However, by principle, the color display is implemented at the great expense of either the contrast or the reflectance. Thus, a display that is both bright and clear cannot be obtained.
As described above, in order to realize a color display while maintaining high contrast, it is necessary to display white color, black color, and the other three color primaries by a single microcapsule. However, an electrophoretic E-ink capable of realizing such a display is not obtained heretofore.
Furthermore, the electrophoresis method above has no distinct threshold voltage, and the problem thereof is that gradation cannot be realized with a single color display unit. That is, in the method above, since few display colors are possible, the display results unclear.
An aspect of the present invention provides an electrophoresis display comprising: a substrate; a first electrode formed on said substrate; capsules provided on said first electrode, and said capsules including a first coloring matter having dichroism, a second coloring matter, and charged particles in a dispersion medium; a second electrode formed on said capsules, wherein said second coloring matter has a smaller dichroism ratio than that of said first coloring matter.
An aspect of the present invention provides an electrophoresis display comprising: a substrate; a first electrode formed on said substrate; a first color capsule provided on said electrode, and including a cyan coloring matter having dichroism and a red coloring matter in a dispersion medium; a second color capsule provided on said electrode, and including a magenta coloring matter having dichroism and a green coloring matter in a dispersion medium; a third color capsule provided on said electrode, and including a yellow coloring matter having dichroism and a blue coloring matter in a dispersion medium; a second electrode formed on said first color capsules, said second color capsules, and said third color capsules, wherein said cyan coloring matter has a smaller dichroism ratio than that of said red coloring matter, said magenta coloring matter has a smaller dichroism ratio than that of said green coloring matter, and said yellow coloring matter has a smaller dichroism ratio than that of said blue coloring matter.
An aspect of the present invention provides an electrophoresis display comprising: means for coloring by a capsule, each of said capsule having three expressible colors; means for impressing a voltage to a capsule.